Concern that legalising gay marriage would mean religious celebrants would be forced to conduct ceremonies, despite their opposition, has been rife since Labour MP Louisa Wall's marriage equality bill was drawn from the member's ballot.

It passed its first reading and is currently before a parliamentary select committee.

Committee chairwoman Ruth Dyson said the committee had considered how to balance the rights of religious, or non-religious, celebrants who did not want to marry gay couples.

''The conversation the committee has had... is that it is not our wish to override that religious freedom. That we do think that is quite fundamental. We haven't made a determination as a committee, that's been the conversation.''

Human Rights Commission chief commissioner David Rutherford told the committee the right to marriage and right to religion could co-exist.

''This is as much about religious freedom in New Zealand as it is about marriage equality.''

The commission said couples should be able to marry regardless of their sexual orientation.

''Same-sex couples and transgender people should be able to have their commitment to each other recognised publicly in the same way as heterosexual couples.

''Not allowing same-sex couples to marry discriminated against people because of their sex or sexual orientation, he said.

''The commission does not consider that access to a civil union provides same-sex couples with full equality.''

The conflict between the right to religious freedom and the right to freedom from discrimination based on sexual orientation was the most contentious issue in the bill.

But supporting gay marriage did not undermine the right to religious belief.

International gay rights activist Boris Dittrich said nobody was suggesting churches or other religious groups should be forced to do anything they did not agree with.

''It's about civil marriage, but you see this big mix-up which happens in all countries... it's up to churches and whether they want to go along or not.''

Father James Lyons, a Catholic priest, told the committee he was opposed to the bill because he believed marriage was about procreation and should be between a man and woman.

''To me this is so central to our social structure.''

National MP Chris Auchinvole said he understood the importance of pro-creation but could not see how gay marriage was a threat to that.

''Sexual activity is a recreational and emotional function as well as just procreation,'' he said.